Thermally stabilized optical preamplifier

ABSTRACT

A thermally stabilized optical preamplifier includes a first transimpedance amplifier which receives a photosignal current from a photodiode. The gate of a GaAs device is the input to the first transimpedance amplifier, and the drain is coupled to a source of constant current. Also coupled to the drain of the GaAs device is a common base stage, and the gain of the first transimpedance amplifier is set by a resistor coupled between the input of the first transimpedance amplifier and the output of the common base stage. A current setting resistor coupled to the output of the first transimpedance amplifier converts output voltage to current, which is summed with a compensating current in a second transimpedance amplifier. The compensating current consists of a variable current generator and a thermal current generator which compensate for the current produced by the absolute value and thermal characteristics of the gate to source voltage of the input GaAs device.

BACKGROUND OF THE INVENTION

This invention relates to optical preamplifiers, and more specificallyto optical preamplifiers which have been compensated in some manner forthe thermal characteristics of an input GaAs device.

High speed circuits such as optical preamplifiers often use GaAs inputdevices to increase operating speed, sometimes in conjunction withsilicon devices. However, the thermal behavior of these devices iscomplex and unrelated to the thermal behavior of silicon junctions.Therefore, what is desired is a circuit which uses a GaAs input devicefor increased operating speeds, yet uses a minimum of conventionalsilicon devices to compensate undesirable thermal coefficients producedby the GaAs device.

SUMMARY OF THE INVENTION

In a preferred embodiment of the present invention, a thermallystabilized optical preamplifier includes a first transimpedanceamplifier which receives a photosignal current from a photodiode. Thegate of a GaAs device is the input to the first transimpedanceamplifier, and the drain is coupled to a source of constant current.Also coupled to the drain of the GaAs device is a common base stage, andthe gain of the first transimpedance amplifier is set by a resistorcoupled between the input of the first transimpedance amplifier and theoutput of the common base stage. A gain setting resistor coupled to theoutput of the first transimpedance amplifier converts output voltage tocurrent, which is summed with a compensating current in a secondtransimpedance amplifier. The compensating current consists of avariable current generator and a thermal current generator whichcompensate for the current produced by the absolute value and thermalcharacteristics of the gate to source voltage of the input GaAs device.

BRIEF DESCRIPTION OF THE DRAWING

For a better understanding of the invention, and to show how the samemay be carried into effect, reference will now be made to theaccompanying drawings, in which the

SOLE FIGURE is a schematic diagram of a thermally stabilized opticalpreamplifier in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The thermally stabilized optical preamplifier 10 shown in the SOLEFIGURE receives a photosignal input current from a photodiode 16 whichis biased between a reference voltage V_(REF1) and the bias voltage(gate to source voltage V_(GS)) of an input GaAs device 18. A highfrequency filter consisting of a small value resistor 12 andcompensating capacitor 14 serve to filter extremely high frequencytransient signals.

A first transimpedance amplifier 15 includes GaAs input device 18,constant current source 20, PNP transistor 22, NPN transistor 26, andresistors 24, 28, and 30. The gate of GaAs input device 18 establishes ahigh input impedance for the first transimpedance amplifier. Biascurrent for the GaAs input device 18 is provided by constant currentsource 20. PNP transistor 22 forms a common base amplifier, with theemitter being coupled to the drain of GaAs input device 18. Additionalopen loop voltage gain is provided by feedback resistor 24. The voltageacross resistor 24 is buffered by the emitter follower amplifierconsisting of transistor 26 and resistor 28 which produces a lowimpedance output at the emitter of transistor 26.

The voltage at the emitter of transistor 26 is equal to the photosignalcurrent of the photodiode 16 times the feedback resistor 30 plus thegate to source voltage, V_(GS), of the GaAs input device 18. Thisundesirable contribution of V_(GS) to the output voltage must becancelled. A current compensation scheme is used to cancel out theeffects of the absolute value of V_(GS) as well as the effects of thethermally dependent value of V_(GS). Before a compensating current canbe added, the output voltage of the first transimpedance amplifier istransformed into a current by current setting resistor 40. Thus, thecurrent flowing through current setting resistor includes a currentportion due to the photosignal current of photodiode 16, as well as aportion due to V_(GS) divided by the feedback resistor 30.

The current compensation circuit 25 consists of a variable currentsource 36, silicon diode 34, and resistors 32 and 38, in conjunctionwith current setting resistor 40 and a second, current summing,transimpedance amplifier 42. Ideally, the undesirable contribution tothe current flowing in resistor 40 due to V_(GS) should be zero.Therefore two currents are provided in a current compensation scheme toeliminate this contribution. A variable current source 36 compensatesthe current contribution of the absolute value of V_(GS) and is summedwith the signal current flowing in current setting resistor 40. Thevalue of variable current source 36 is simply adjusted until thecontribution of the absolute value of V_(GS) is zero. Once this has beenaccomplished, the thermally dependent current due to V_(GS) is minimizedwith a thermal current generator 35 consisting of silicon diode 34,resistor 32, and thermal setting resistor 38.

The following equations and assumptions are useful for determining thevalues of the current setting resistor 40 and the thermal settingresistor 38 in order that the unwanted thermal current contribution isminimized. ##EQU1##

For the thermal current contribution to be zero, the following conditionmust be satisfied: ##EQU2##

To minimize the thermal current contribution of V_(GS), it has beenshown that the negative of the thermal gradient of silicon diode 34 mustbe set to the thermal gradient of the gate to source voltage of GaAsdevice 18, measured at the emitter of transistor 26, times the ratio ofresistor R1 (R₄₀) divided by resistor R2 (R₃₈). The currentcontributions of the first transimpedance amplifier 15 and thecompensation current generator 25 are summed by the secondtransimpedance amplifier 42 and a voltage output is provided at terminal44 which is proportional only to the photosignal current of photodiode16.

Typical values of the components of the SOLE FIGURE include 8.6 voltsfor V+, -8.6 volts for V-, 4.3 volts for V_(REF2), -2.5 volts forV_(REF3), 4.7K ohms for R₂₄, 1K ohms for R₂₈, and 1K ohms for R₄₀.Feedback resistor 30 may be comprised of several parallel resistorswhich are selectively switched for alternative gain modes. The value ofR₃₈ and R₄₀ may be selected in accordance with the equations provided,in order that undesirable temperature effects are minimized. Othercomponents may be of any suitable commercial type. As in any high speedcircuit, care must be taken that the power supplies are decoupled toprevent oscillations and improve high frequency performance.

It will be appreciated that the present invention is not restricted tothe particular embodiment that has been described, and that variationsmay be made therein without departing from the scope of the invention asdefined in the appended claims and equivalents thereof.

I claim:
 1. A thermally stabilized optical preamplifier comprising:(a)means for producing a photosignal current in response to an externallight source; (b) a first transimpedance amplifier including an inputGaAs device coupled to the producing means, the first transimpedanceamplifier having a voltage output, the voltage output being the sum ofthe bias voltage of the GaAs device and a voltage proportional to thephotosignal current; (c) means for converting the voltage output of thefirst transimpedance amplifier to a signal current; (d) a secondtransimpedance amplifier having a voltage output and a current summinginput for receiving the signal current; and (e) means for providing acompensation current to the current summing input of the secondtransimpedance amplifier, the compensation current substantiallycancelling the portion of the signal current produced by the biasvoltage of the GaAs device.
 2. A thermally stabilized opticalpreamplifier as in claim 1 wherein the first transimpedance amplifiercomprises:a GaAs device having a drain, a source, and a gate, the gateforming the current input, the drain being coupled to a constant currentsource, and the source being coupled to AC ground; a common base gainstage having an input and an output, the input being coupled to thedrain of the GaAs device and the output providing the voltage output;and a feedback resistor coupled between the gate of the GaAs device andthe output of the common base gain stage.
 3. A thermally stabilizedoptical preamplifier as in claim 1 wherein the means for providing acompensation current comprises a thermal current generator having acurrent output and a variable current generator having a current output,the current outputs being coupled together to produce the compensationcurrent.
 4. A thermally stabilized optical preamplifier as in claim 3wherein the thermal current generator comprises:a silicon junctionvoltage generator having a voltage output; and a thermal settingresistor coupled between the output of the silicon junction voltagegenerator and the current summing input of the second transimpedanceamplifier.
 5. A thermally stabilized optical preamplifier as in claim 4wherein the value of the current setting resistor is R1, the value ofthe thermal setting resistor is R2, the derivative of the voltage outputof the silicon junction voltage generator is K1, the derivative of thebias voltage of the GaAs device is K2, and R1 and R2 are selected suchthat R1K1=-R2K2.